GROWN UP: Linda Cardellini (above) is Don Draper’s new side-squeeze on “Mad Men”. But you may remember her as tomboyish Lindsay on “Freaks and Geeks”.

GROWN UP: Linda Cardellini is Don Draper’s new side-squeeze on “Mad Men” (above). But you may remember her as tomboyish Lindsay on “Freaks and Geeks”.

GROWN UP: Linda Cardellini is Don Draper’s new side-squeeze on “Mad Men”. But you may remember her as tomboyish Lindsay on “Freaks and Geeks” (above).

GROWN UP: Linda Cardellini (above) is Don Draper’s new side-squeeze on “Mad Men”. But you may remember her as tomboyish Lindsay on “Freaks and Geeks”. (
)

GROWN UP: Linda Cardellini (center) is Don Draper’s new side-squeeze on “Mad Men” (inset). But you may remember her as tomboyish Lindsay on “Freaks and Geeks” (right). (
)

Don Draper couldn’t stay faithful forever, right?

As the final scene of Sunday’s “Mad Men” premiere revealed, the Madison Avenue philanderer didn’t have to travel very far to satisfy his wanderlust.

Draper’s downstairs neighbor, Mrs. Sylvia Rosen, wife of his alleged friend, Dr. Arnie Rosen, was waiting for Don’s knock on the service door to her high-rise apartment after her husband left the Drapers’ New Year’s Eve party for a medical emergency.

Cut to the philanderers laying in bed and not exactly basking in the afterglow.

“What do you want for the New Year?” Sylvia asked.

“I want this to stop,” said the always unsatisfied Don.

“I know,” Sylvia said resignedly.

Draper’s new mistress is played by actress Linda Cardellini, who periodically pops up in interesting roles. After a guest appearance as Lauren, the girl who broke up Cory and Topanga, on the 1998 ABC series “Boy Meets World,” she was cast as tomboy Lindsay Weir in the landmark Judd Apatow and Paul Feig comedy series “Freaks and Geeks” in 1999.

Cardellini, now 37, co-starred on the NBC show, which only lasted one season, with future stars James Franco, Seth Rogen and Jason Segel.

She won good notices for her role as nurse Samantha Taggart on “ER” in 2003, and stayed with the NBC ratings hit for six years until its finale.

Cardellini took time out from her “ER” duties to play a small-but-pivotal role in Ang Lee’s “Brokeback Mountain” as Cassie, a cowgirl who briefly dates Heath Ledger’s closeted cowboy, Ennis Del Mar.

Her association with “Mad Men” has been top-secret. She said in Entertainment Weekly that not even her best friends and family knew she was doing the show. She wears a 1960s-style short black wig for the part, whose importance in the developments of Season 6 has not been disclosed.